There is considerable interest in chemical systems that store and release gases such as hydrogen. Such systems are expected to find large-scale use as hydrogen fuel cells in a variety of applications. To date, two main chemical approaches have been employed to store hydrogen (See Schlapbach, L., Züttel, A. Nature 2001, 414, 353). The first approach involves adsorption of hydrogen within low-density porous materials. In this approach, carbonaceous (e.g. nanotubes) and silicon-rich (e.g. zeolites) solids have been shown to exhibit appreciable sorption capacities. The second approach involves absorption of hydrogen by reactive high-density materials such as metal hydrides. In this process, a hydride interacts with hydrogen to cause dissociation of the molecule. Two hydrogen atoms recombine to facilitate the desorption process.
Although the above approaches are promising for the development of viable hydrogen storage systems, each has significant drawbacks. Despite offering a promise of low-density, lightweight storage materials, adsorption systems have been based on structures that are either not well defined (e.g. nanostructured carbon) or possess hydrophilic surfaces that do not interact favorably with hydrogen (e.g. zeolite). High pressures are also typically required to facilitate the adsorption process. In the case of metal hydrides, the high densities of such solids have been deemed unacceptable for many practical purposes.
Additionally, U.S. Pat. No. 4,359,327 reports the use of certain specific cyclophanes to store hydrogen. However, the use of this technology is limited because the cyclophane must be dissolved in a liquid (e.g. organic solvent) to facilitate the absorption process.
Porous crystalline solids that employ metal-organic components as building blocks, where a rigid, linear organic bridge propagates the coordination geometry of a metal node in one- (1D), two- (2D), or three- (3D) dimensions, are attracting much interest. (Recent representative examples include J. S. Seo, et al., Nature 2000, 404, 982-986; S. M.-F. Lo, et al., J. Am. Chem. Soc. 2000, 122, 6293-6294; H. Li, et al., Nature 1999, 402, 276-279; B. Moulton, et al., Chem. Commun. 1999, 1327-1328; S. W. Keller, S. Lopez, J. Am. Chem. Soc. 1999, 121, 6306-6307; J. D. Ranford, et al., Angew. Chem. 1999, 111, 3707-3710 and Angew. Chem. Int. Ed. 1999, 38, 3498-3501; L. R. MacGillivray, et al., J. Am. Chem. Soc. 1998, 120, 2676-2677; M. J. Zaworotko, Angew. Chem. 2000, 112, 3180-3182 and Angew. Chem. Int. Ed. Engl. 2000, 39, 3052-3054; D. Venkataraman, et al., J. Am. Chem. Soc. 1995, 117, 11600-11601; C. J. Kepert, M. J. Rosseinsky, Chem. Commun. 1999, 375-376; K. Biradha, et al., Angew. Chem. 2000, 112, 4001-4003; and Angew. Chem. Int. Ed. Engl. 2000, 39, 3843-3845).
Such metal-organic frameworks (MOFs) are designed to exhibit properties that mimic, and improve upon, more conventional porous solids such as zeolites (J. V. Smith, Chem. Rev. 1988, 88, 149-182) and mesoporous materials (MCMs) (Selvam, S. K. Bhatia, C. G. Sonwane, Ind. Eng. Chem. Res. 2001, 40, 3237-3261).
Many porous MOFs, however, have fallen short, in contrast to zeolites and MCMs, as robust porous solids. (M. J. Zaworotko, Angew. Chem. 2000, 112, 3180-3182; Angew. Chem. Int. Ed. Engl. 2000, 39, 3052-3054; S. R. Batten, R. Robson, Angew. Chem. 1998, 110, 1558-1595; Angew. Chem, Int. Ed. Engl. 1998, 37, 1460-1494). Interpenetration and framework fragility have hampered progress such that host cavities tend to self-include while guest removal often results in a collapse of host structure. (S. R. Batten, R. Robson, Angew. Chem. 1998, 110, 1558-1595; Angew. Chem. Int. Ed. Engl. 1998, 37, 1460-1494; M. J. Zaworotko, Angew. Chem. 2000, 112, 3180-3182; Angew. Chem. Int. Ed. Engl. 2000, 39, 3052-3054).
Recently, however, such problems of interpenetration and framework fragility have been largely circumvented using metal clusters, as secondary building units (SBUs), for host design (M. Eddoudi, et al., Acc. Chem. Res. 2001, 34, 319-330). SBUs (e.g. metal-carboxylates) reduce the likelihood of interpenetration owing to their large sizes which can preclude filling of void spaces, producing stable, porous solids able to support inclusion and catalysis (J. S. Seo, et al., Nature 2000, 404, 982-986; S. M.-F. Lo, et al., J. Am. Chem. Soc. 2000, 122, 6293-6294; H. Li, et al., Nature 1999, 402, 276-279).
Although SBUs have been successfully employed for the construction of MOFs with stable pores, it can be difficult, in contrast to MCMs to line the interiors of such solids with organic groups since an elaborate covalent synthesis of a linear organic bridge is often required to introduce simple (e.g. —Me) and diverse (e.g. chiral) functionalities. (Selvam, S. K. Bhatia, C. G. Sonwane, Ind. Eng. Chem. Res. 2001, 40, 3237-3261; MCMs are readily functionalized with organic groups by post-synthetic graphting: See: M. H. Lim, et al., J. Am. Chem. Soc. 1997, 119, 4090-409).
Accordingly, there is currently a need for new chemical systems that can be used to store gases (e.g. hydrogen). Such systems would be useful in numerous applications, such as, for example, in the manufacture of fuel cells.